Water Quality Ambient Water Quality Criteria for Dissolved Oxygen Water Quality Ambient Water ...

89 Pages·2016·0.83 MB·English

by  Brian Beitz

Water Quality
Ambient Water Quality Criteria for Dissolved Oxygen

Water Management Branch
Environment and Lands Headquarters Division
Ministry of Environment, Lands and Parks
Prepared pursuant to Section 2(e) of the
Environment Management Act, 1981
February 1997

Water Quality
Ambient Water Quality Criteria for Dissolved Oxygen

1.0 Introduction
Oxygen is the single most important component of surface water for self-purification processes and the
maintenance of aquatic organisms which utilize aerobic respiration. This document is largely biased
towards the effects of minimum oxygen levels on aquatic life, as dissolved oxygen has relatively little or
no consequence for other water uses. Categories of water uses given cursory consideration include
drinking water, recreation and aesthetics, and industry. A discussion of dissolved oxygen in the context
of criteria (acceptable levels for specifies uses) is anomalous relative to most other water quality
variables in that critical oxygen levels are expressed as minimum rather than maximum values. An
exception is required for industry because of the corrosive properties of oxygen.
Dissolved oxygen standards and criteria from other agencies and jurisdictions are reviewed along with
information available from the literature. The objective or the review was to incorporate the most
applicable information which could be used to formulate defensible criteria to protect aquatic life in British
Columbia waters. Criteria for other water uses have not been proposed.
WATER QUALITY

Ambient Water Quality Criteria for Dissolved Oxygen

Ministry of Environment Water Protection and Sustainability Branch Mailing Address: Telephone: [phone]
Environmental Sustainability PO Box 9362 Facsimile: 250 356-1202
and Strategic Policy Division Stn Prov Govt Website: www.gov.bc.ca/water

Victoria BC V8W 9M2
2.0 Forms and Transformations
2.1 Physical Properties
Oxygen is the most abundant element of the earth's crust and waters combined. The combination of the
divalent oxygen atom with single valent hydrogen atom comprises the extremely stable H O molecule.
2
Under natural conditions water exists in several physical states, but the molecule itself dissociates to a
very limited extent as ions (H+ and OH-). Two OH- molecules can, by covalent bonding, combine to form
H O or hydrogen peroxide. Holm et al. (1986) state that there is evidence that hydrogen peroxide is
2 2
formed and accumulates in the photo-oxidation of organic compounds in surface and ground waters and
in precipitation.
The decomposition of water to yield dissolved oxygen normally would be outside the realm of ambient
conditions; an endothermic reaction such as provided by electrolysis is required to produce O and H
2 2
gas. Photosynthesis is the only natural process that oxidizes water to oxygen. Another reaction of
oxygen atoms is the formation of ozone (O ), which occurs naturally when mediated by absorption of
3
ultraviolet light, or can be manufactured artificially using high electrical voltage. Ozone is highly unstable
and is normally confined to the upper atmosphere. Here, there is sufficient intensity of ultraviolet light to
split the stable oxygen molecules, freeing oxygen atoms and promoting recombination with other
molecules to form ozone. Substantial qualities of ozone are increasingly being found in areas where air
quality is degraded. Ground-level ozone is formed by the reaction of byproducts from fossil fuel
combustion (hydrocarbons and nitrogen oxides) in the presence of sunlight.
The double bonded, two-atom molecule is the single form of oxygen which has relevance to this
discussion. Air contains approximately 20.9 percent oxygen gas by volume; however, the proportion of
dissolved oxygen in air dissolved in water is about 35 percent, because nitrogen (the remainder) is less
soluble in water. Oxygen is considered to be moderately soluble in water and this solubility is governed
by a complex set of physical conditions that include atmospheric and hydrostatic pressure, turbulence,
temperature and salinity (Wetzel, 1983). A brief description follows of how these conditions relate to and
influence dissolved oxygen.

Atmospheric and hydrostatic pressure-Henry's Law states that the amount of oxygen which will remain
dissolved in a volume of water, at constant temperature, is proportional to the ambient pressure of
oxygen gas with which it is in equilibrium (CREM, 1987). Air pressure at sea level under standard
conditions (fully saturated with oxygen and water vapour, 0 degrees Celsius) is equal to 760 mm Hg (or
101.325 kilopascals) and the proportion of this pressure attributable to oxygen is directly related to the
fraction of oxygen in air. Oxygen tension or partial pressure (PO ) is equivalent to atmospheric pressure
2
minus a compensation factor for water vapour pressure (the latter is available in tables as in Colt, 1984),
multiplied by the oxygen fraction in air:
PO = (Atmos. Press. - Water Vapour Press.) x % O
2 2
Davis (1975) presented the following example at 10 degrees Celsius and one atmosphere (sea level):
PO = (760 mm Hg - *9.2 mm Hg) x 20.95/100
2
= 157.3 mm Hg
* saturated water vapour pressure at 10 degrees Celsius (from Table 1)
Thus, at any given barometric pressure and temperature (and corresponding water vapour pressure) the
oxygen partial pressure can be calculated. At altitudes above sea level the gravitational attraction of gas
molecules becomes less and there is a progressive reduction in barometric pressure. Tables are
available (e.g. Table 2 from NCASI, 1985) which provide correction factors for computations of oxygen
partial pressure at altitude.
For criteria purposes it is more common to express oxygen content in terms of concentration rather than
partial pressure. This concentration usually is represented by solubility in mg/L (ppm) or mL/L and these
units have corresponding pressure equivalents. In the previous example with freshwater at 10 degrees
Celsius and 157.3 mm Hg, the air-equilibrated solubility is 7.90 mL/L or 11.29 mg/L from solubility tables
(e.g., Table 3 from APHA, 1992-column one for freshwater). If this sample was 50 percent saturated, the
concentration and pressure equivalents would simply be halved (Davis, 1975). The oxygen solubility
values in Table 3 represent full (100 percent) saturation of oxygen under one set of conditions.
For barometric pressures other than 760 mm Hg (sea level), oxygen solubilities can be computed from
the following equation:
C* = C*760(P - p) / (760 - p) (from Colt, 1984)
t
C* = oxygen solubility
C*760 = saturation value at 760 mm Hg (Table 3)
P= barometric pressure (mm Hg)
t
p = vapour pressure of water (Table 1)
Example: Give the oxygen solubility at 15 degrees Celsius when the barometric pressure is 29.33 in mm
Hg.
P = 29.33 in Hg (25.4 mm/in) = 745 mm Hg
t
p = 12.79 mm Hg (form Table 1)
C* = 10.08 mg/L(745 mm Hg - 12.79 mm Hg) / (760 mm Hg - 12.79)
= 9.88 mg/L

Table 1. Vapour Pressure of Freshwater in mm Hg as a Function of Temperature
Temp.C 0.0 0.1 0.3 0.4 0.5 0.6 0.7 0.8 0.9
0 4.58 4.62 4.65 4.68 4.72 4.75 4.79 4.82 4.86 4.89
1 4.93 4.96 5.00 5.04 5.07 5.11 5.14 5.18 5.22 5.26
2 5.29 5.33 5.37 5.41 5.45 5.49 5.53 5.57 5.60 5.64
3 5.68 5.73 5.77 5.81 5.85 5.89 5.93 5.97 6.02 6.06
4 6.10 6.14 6.19 6.23 6.27 6.32 6.36 6.41 6.45 6.50
5 6.54 6.59 6.64 6.68 6.73 6.78 6.82 6.87 6.92 6.97
6 7.01 7.06 7.11 7.16 7.21 7.26 7.31 7.36 7.41 7.46
7 7.51 7.57 7.62 7.67 7.72 7.78 7.83 7.88 7.94 7.99
8 8.05 8.10 8.16 8.21 8.27 8.32 8.38 8.44 8.49 8.55
9 8.61 8.67 8.73 8.79 8.85 8.91 8.97 9.03 9.09 9.15
10 9.21 9.27 9.33 9.40 9.46 9.52 9.59 9.65 9.72 9.78
11 9.85 9.91 9.98 10.04 10.11 10.18 10.24 10.31 10.38 10.45
12 10.52 10.59 10.66 10.73 10.80 10.87 10.94 11.01 11.09 11.16
13 11.23 11.31 11.38 11.46 11.53 11.61 11.68 11.76 11.83 11.91
14 11.99 12.07 12.15 12.23 12.30 12.38 12.46 12.55 12.63 12.71
15 12.79 12.87 12.96 13.04 13.12 13.21 13.29 13.38 13.46 13.55
16 13.64 13.73 13.81 13.90 13.99 14.08 14.17 14.26 14.35 14.44
17 14.53 14.63 14.72 14.81 14.91 15.00 15.10 15.19 15.29 15.38
18 15.48 15.58 15.68 15.78 15.88 15.97 16.08 16.18 16.28 16.38
19 16.48 16.59 16.69 16.79 16.90 17.00 17.11 17.22 17.32 17.43
20 17.54 17.65 17.76 17.87 17.98 18.09 18.20 18.31 18.43 18.54
21 18.66 18.77 18.89 19.00 19.12 19.24 19.36 19.47 19.59 19.71
22 19.83 19.96 20.08 20.20 20.32 20.45 20.57 20.70 20.82 20.95
23 21.08 21.20 21.33 21.46 21.59 21.72 21.85 21.99 22.12 22.25
24 22.39 22.52 22.66 22.79 22.93 23.07 23.21 23.34 23.48 23.63
25 23.77 23.91 24.05 24.19 24.34 24.48 24.63 24.78 24.962 25.07
26 25.22 25.37 25.52 25.67 25.82 25.98 25.13 26.28 26.44 26.59
27 26.75 26.91 27.07 27.23 27.39 27.55 27.71 27.87 28.03 28.20
28 28.36 28.53 28.69 28.86 29.03 29.20 29.37 29.54 29.71 29.88
29 30.06 30.23 30.41 30.58 30.76 30.94 31.12 31.30 31.48 31.66
30 34.84 32.02 32.21 32.39 32.58 32.77 32.95 33.14 33.33 33.52
31 33.71 33.91 34.10 34.29 34.49 34.69 34.88 35.08 35.28 35.48
32 35.68 35.89 36.09 36.29 36.50 36.70 36.991 37.12 37.33 37.54
33 37.75 37.96 38.18 38.39 38.61 38.82 39.04 39.26 39.48 39.70
34 39.92 40.14 40.37 40.59 40.82 41.05 41.28 41.51 41.74 41.97
35 42.20 42.43 42.67 42.91 43.14 43.38 43.62 43.86 44.10 44.35
36 44.59 44.84 45.08 45.33 45.58 45.83 46.08 46.33 46.59 46.84
37 47.10 47.35 47.61 47.87 48.13 48.40 48.66 48.92 49.19 49.46
38 49.72 49.99 50.27 50.54 50.81 51.09 51.36 51.64 51.92 52.20
39 52.48 52.76 53.04 53.33 53.62 53.90 54.19 54.48 54.78 55.07
40 55.36 55.66 55.96 56.25 56.55 56.86 57.16 57.46 57.77 58.07
Source: Colt, 1984

Tabulated oxygen saturation values are available as a function of barometric pressure and elevation over
a range of temperatures (e.g., as in Colt, 1984). The correction factors, listed in Table 2, also can be
applied directly to oxygen solubilities at the range of elevations shown (this inverse relationship is linear).
For non-standard pressures and elevations, Wetzel (1983) provides a nomogram from which oxygen
solubility and percent saturation can be derived at an observed temperature and oxygen content. In
special cases, when the composition of dissolved gases under study differs from that of air, Bunsen
coefficients can be used to calculate solubilities (mole fractions of gases must be known); Colt (1984)
provides the necessary formulae and tables for these calculations.
At any particular depth in a column of water, the amount of gas that can be held in solution is determined
by the combined atmospheric and hydrostatic pressures, and is known as the absolute saturation
(Wetzel, 1983). Normally, saturation is considered in relation to the pressure at the water's surface, at a
specific temperature and salinity. Supersaturation, a non-equilibrium situation, is the term used when the
partial pressures of gasses (primarily nitrogen and oxygen) in solution exceed their equivalent
atmospheric pressures. Hydrostatic pressure increases rapidly with depth and dissolved gas solubility
doubles approximately every 10 m (hence, the increased efficiency of aeration devices at depth), while
the degree of supersaturation decreases with depth (Colt, 1984). For example, a gas supersaturation of
130 percent (surface measurement) is reduced to 100 percent saturation at a depth of 3.0 m.

Table 2. Correction Factors for Barometric Pressure and Oxygen Saturation at Altitude
ALTITUDE CORRECTION
(feet) (metres) FACTOR
0 0 1.00
500 152 0.98
1000 305 0.96
1500 457 0.95
2000 610 0.93
2500 762 0.91
3000 914 0.89
3500 1067 0.88
4000 1219 0.86
4500 1372 0.84
5000 1524 0.82
5500 1676 0.81
6000 1829 0.80
Notes:
1. Multiply barometric pressure of dissolved oxygen solubility at sea level for the appropriate temperature
(Table 3) by the correction factor for your altitude.
2. Interpolate, using linear relationship, for greater accuracy.Source: NCASI, 1985.

Table 3. Solubility of Oxygen in Water (Fresh and Saline) Exposed to Water-saturated Air
at Sea Level (760 mm Hg (101.3 kPa)
Oxygen Solubility
(mg/L)
Temp. Chlorinity (freshwater)
(C) 0 5.0 10.0 15.0 20.0 25.0
0.0 14.621 13.728 12.888 12.097 11.355 10.657
1.0 14.216 13.356 12.545 11.783 11.066 10.392
2.0 13.829 13.000 12.218 11.483 10.790 10.139
3.0 13.460 12.660 11.906 11.195 10.526 9.897
4.0 13.107 12.335 11.607 10.920 10.273 9.664
5.0 12.770 12.024 11.320 10.656 10.031 9.441
6.0 12.447 11.727 11.046 10.404 9.799 9.228
7.0 12.139 11.442 11.783 10.162 9.576 9.023
8.0 11.843 11.169 10.531 9.930 9.362 8.826
9.0 11.559 10.907 10.290 9.707 9.156 8.636
10.0 11.288 10.656 10.058 9.493 8.959 8.454
11.0 11.027 10.415 9.835 9.287 8.769 8.279
12.0 10.777 10.183 9.621 9.089 8.586 8.111
13.0 10.537 9.961 9.416 8.899 8.411 7.949
14.0 10.306 9.747 9.218 8.716 8.242 7.792
15.0 10.084 9.541 9.027 8.540 8.079 7.642
16.0 9.870 9.344 8.844 8.370 7.922 7.496
17.0 9.665 9.153 8.667 8.207 7.770 7.356
18.0 9.467 8.969 8.497 8.049 7.624 7.221
19.0 9.276 8.792 8.333 7.896 7.483 7.090
20.0 9.092 8.621 8.174 7.749 7.346 6.934
21.0 8.915 8.456 8.021 7.607 7.214 6.842
22.0 8.743 8.297 7.873 7.470 7.087 6.723
23.0 8.578 8.143 7.730 7.337 6.963 6.609
24.0 8.418 7.994 7.591 7.208 6.844 6.498
25.0 8.263 7.850 7.457 7.083 6.728 6.390
26.0 8.113 7.711 7.327 6.962 6.615 6.285
27.0 7.968 7.575 7.201 6.845 6.506 6.184
28.0 7.827 7.444 7.079 6.731 6.400 6.085
29.0 7.691 7.317 6.961 6.621 6.297 5.990
30.0 7.559 7.194 6.845 6.513 6.197 5.896
Notes:
1. Formulae are available for equilibrium oxygen concentration at non-standard pressures and for all
chlorinity values.
2. For wastewater, it is necessary to know the ions responsible for the solution's electrical conductivity to
correct for their effect on oxygen solubility and use of the tabular value.
Source: APHA, 1992

The degree of oxygen supersaturation necessary for bubble growth increases with depth. Ramsey
(1962) explained that, in the absence of turbulence, bubbles may form due to the partial pressure of
oxygen at depths of less than one metre. Below four metres, oxygen will be maintained in solution by
hydrostatic pressure even when extremely supersaturated relative to the pressure at the surface.
The entrainment of air below water falls or dam spillways is a common cause of supersaturation that first
came to prominence as an environmental problem in the Pacific Northwest in the Columbia River system
(primarily in Washington State, but also below the Hugh Keenleyside Dam near Castlegar). Gas bubbles
can develop in fish and invertebrates due to a large imbalance between ambient and internal partial
pressures, and lethal or sublethal effects can result. Since oxygen usually is not the principal gas of
importance (nitrogen is) and total gas pressure is more central to the issue of supersaturation, gas
bubble disease is dealt with in a separate criteria document on total dissolved gases. The effect of
hydrostatic pressure also must be taken into account when measuring oxygen concentrations at great
depths by an electrode as opposed to a chemical technique. Oxygen solubility remains effectively
constant with depth whereas partial pressure increases; therefore, a polarographic probe which
measures partial pressure rather than concentration must be corrected accordingly. For example, at 400
m a correction of 5 percent (less) is necessary (Hitchman, 1978).
Turbulence - the diffusion of gas in water is slow and, for equilibrium with atmospheric oxygen to be
established, circulation must occur such as in the epilimnion of stratified lakes or at periods of turnover.
The rate of oxygen distribution and equilibration is dependent on turbulence. Increased turbulence forms
a greater surface area from which excess gasses from supersaturation can dissipate, and brings trapped
subsurface water to the surface (NCASI, 1985). In cases where the initial dissolved oxygen
concentrations at depth are not far from saturation, equilibrium may occur in a few days. Alternatively, in
deep lakes complete oxygenation may never be achieved before thermal stratification terminates
circulation for a seasonal interval (Wetzel, 1983). Oxygen distribution will be discussed further in Section
3.1.
Temperature -Temperature, more than any other physical condition in the aquatic environment, affects
the solubility potential of dissolved oxygen. This relationship is non-linear as solubility increases
considerably in cold water (Wetzel, 1983). Freshwater is saturated with 14.6 mg O2/L at 0 degrees
Celsius, which declines to 8.3 mg O2/L at 25 degrees Celsius (at sea level). As solubility declines with
increased water temperature, Davis (1975) points out that oxygen partial pressure drops only slightly due
to increased molecular activity.
Oxygen solubility tables for a range of temperatures are available from a number of sources; however,
references prior to 1981 should be avoided due to updating of these solubilities. Table 3 was extracted
from a larger table in APHA (1992) which lists solubility values for dissolved oxygen in freshwater and
saline waters, equilibrated with air at one atmosphere (sea level).
Salinity -The oxygen content of water decreases exponentially as salinity increases, such that the
difference between solubility in seawater and freshwater is about 20 percent (Wetzel, 1983). Tables
(e.g., Table 3) and nomograms (e.g. Figure 1) are available for deriving oxygen saturation in saline
waters. The new definition of salinity, which was adopted by the Standard Methods Committee in 1985,
is based on the electrical conductivity of seawater. Specific conductance is converted against a known
standard (KCl in water) to chlorinity and then to total salinity by a correction factor:
salinity = 1.80655 x chlorinity
The scale has no dimensions, therefore parts per thousand (g/kg) no longer applies (APHA, 1989).

Figure 1. Nomogram of Oxygen Solubility in Air-saturated Water at Different Salinities
Source: Hitchman, 1978

2.2 Analytical Methods
2.2.1 Surface Water
There are two common methods for determining the solubility of oxygen in water: the Winkler or
iodometric method and its modifications, and the electrometric method using membrane electrodes. The
precision of other chemical and colorimetric methods is invariably less than that for the Winkler method
(Hitchman, 1978). The Winkler method involves the more precise titrimetric procedure based on the
oxidizing property of dissolved oxygen, while the membrane electrode procedure is based on the rate of
diffusion of molecular oxygen across a membrane (APHA, 1992). Since the amount of oxygen in water is
dependent upon a complex set of physical properties and biological processes, the method of
measurement must be suited to the source water. Temperature, salinity, turbulence, pressure,
photosynthetic activity, respiration and chemical interferences (oxidizing or reducing compounds) affect
the concentration of dissolved oxygen in water.
Iodometric Procedures - APHA (1992) describes four derivations of the Winkler method, the selection
of which is based on minimizing the effects of interfering materials known to be present. For example,
the azide modification for most effluent and stream measurements removes interferences caused by
nitrite, the most common interference in biologically-treated effluents. Zenon Environmental Laboratories
uses this method (reagents include manganese sulphide, potassium salt and sulphuric acid) for
calibrating oxygen meters. A determination of 0.05 mg/L is possible, which can ensure a meter accuracy
of 0.1 mg/L (Heier, 1991). The other procedures described in APHA (1992) include the permanganate
modification used for samples containing ferric and ferrous iron (e.g., acid mine drainage), the alum
flocculation modification which removes interferences from high suspended solids, and the copper
sulphate-sulfamic acid flocculation modification for biological flocs (e.g., activated sludge) which have
high oxygen utilization rates. Further modifications are available for the following: Pomeroy-Kirschman
method when high dissolved oxygen levels (> 15 mg/L) or high organic content are present, Alkali-
hypochlorite modification in the presence of SO32-, S2 O32-, and polythionate, and the "Short"
modification for organic substances which are readily oxidized in strong alkali or by the iodine in acid
solution (Hitchman, 1978). A major disadvantage of the above methods is that they are not appropriate
for in situ measurements. Samples should be handled carefully to avoid agitation and contact with air,
and special equipment is necessary to eliminate changes in pressure and temperature when sampling at
depth. It is commonly acknowledged that dissolved oxygen is best measured in the field because of the
changes in concentration that are likely to occur between sampling and lab analysis. In some instances,
fixative agents (including sulphuric acid, sodium azide) can be used by collectors to stabilize samples for
transit to a lab, but these chemicals are costly and extremely corrosive and accuracy of the dissolved
oxygen determination still would be questionable. Equipment for measuring oxygen levels in the field is
described in the following sections.

Electrometric Procedures - Early oxygen sensors had to be designed for each analytical situation, and
electrodes used were subject to direct exposure to the sample medium. The most significant
development in the design of efficient sensors was achieved by Dr. Leland Clark whose membrane-
covered electrode reduced the risk of contamination and provided a more uniform diffusion layer for
oxygen to pass. Present generation meters are a convenient size, simple to operate and reasonably
rugged. The submersible electrodes are particularly useful for continuous monitoring, profiling dissolved
oxygen with depth and testing waters which have high interference values (effluents, particulates, colour,
etc.). Zenon Laboratories, for example, uses an Orion meter for conducting continuous dissolved oxygen
analyses for biochemical oxygen demand (Heier, 1991). Some of the newest models incorporate
computerized remote control and interfacing to download data (e.g., YSI Model 59). There are three
variations of membrane-covered probes commonly in use, each having specific attributes. Figure 2
contains schematic diagrams and probe reactions as examples of galvanic, polarographic and oxygen
balance sensors.
The galvanic sensor is self-polarizing and produces its own electric current. A lead or silver anode and a
silver cathode reside within a potassium hydroxide electrolyte, and galvanic potential is produced by the
reduction of oxygen at the cathode. The current generated is proportional to the rate of oxygen diffusion
through the membrane (which is dependent on the concentration of molecular oxygen) (YSI, 1989).
The most common sensor is the polarographic probe, which employs a silver anode and gold cathode
in a potassium chloride electrolyte. When voltage is applied, oxygen accepts electrons from the cathode.
For each molecule that is reduced, a proportional current is registered that is converted to oxygen
content.
A newer and more sophisticated system is employed in oxygen balance sensors, which were designed
to address some of the shortcomings of the previous probes. Three electrodes (or more) operate in a
potassium hydroxide electrolyte. Oxygen still defuses through a membrane and is reduced at the
cathode(s); however, an equal quantity is generated at the anode(s). This diffusion continues until the